How much water does corn need per week?

Mid-season corn (tasseling through silking) typically needs 1.5 to 2.5 inches of water per week depending on weather. At a Reference ET of 1.8 in/week and Kc of 1.20, crop ET = 2.16 in/week. After subtracting any rainfall, the net irrigation requirement is usually 1.0 to 2.0 inches per week during the July peak. In acre-inch terms, that is 27,000 to 54,000 gallons per acre per week at the irrigation system outlet.

What is the difference between net and gross irrigation?

Net irrigation is the water the crop actually needs — the deficit after rainfall. Gross irrigation is the amount you must pump and apply to deliver that net requirement, accounting for system losses. A center pivot at 85% efficiency requires 1.18 inches gross to deliver 1.0 inch net. Gross = Net divided by System Efficiency. The difference (18%) represents water lost to wind drift, evaporation from the soil surface, and runoff.

How does drip irrigation save water compared to center pivot?

Drip irrigation is 90–95% efficient vs. 80–85% for center pivot. For every 1.0 inch of net irrigation need, a drip system applies roughly 1.05–1.11 inches gross while a pivot applies 1.18–1.25 inches gross — roughly 10–15% less water for the same agronomic result. On a 50-acre field with a 2-inch net need, that difference can be 2.7 to 5.4 acre-feet per week that the drip system saves.

What is a reference ET and how do I find it for my area?

Reference ET (ETo) is the evapotranspiration from a standardized 12-cm grass surface under your local weather conditions, computed using the FAO Penman-Monteith equation. Free sources by region: CIMIS (California), Kansas/Oklahoma/Texas Mesonets (Southern Plains), AgriMet (Pacific NW), Midwest Climate Center (Corn Belt), and USDA NRCS. Your local Cooperative Extension office also publishes weekly ETo summaries during the growing season.

What does the crop coefficient (Kc) mean?

The crop coefficient (Kc) is the ratio of actual crop ET to reference ET. It accounts for how a given crop's canopy size, leaf area, stomatal behavior, and ground cover differ from the reference grass surface. Kc = 1.0 means the crop uses the same water as reference grass; Kc > 1.0 (like corn at 1.20) means the crop uses more; Kc < 1.0 (like sparse early-season crops) means it uses less. Kc values come from FAO-56 field research.

How do I estimate my seasonal water budget?

The seasonal budget is approximately the sum of weekly gross irrigation over the entire growing season. For corn, peak demand lasts 6–8 weeks but the full season runs 18–22 weeks. A rule of thumb for irrigated corn in the Corn Belt is 12–16 acre-inches of gross applied water per season, equivalent to 325,000 to 435,000 gallons per acre depending on rainfall. Use the Seasonal Schedule tab for a month-by-month breakdown.

Can I use this calculator for lawn and turf irrigation?

Yes — select Grass/Lawn as the crop type. Turf grass has a nearly constant Kc of 0.85 year-round because mowing maintains a consistent leaf area. The main variable is Reference ET, which changes dramatically by season and climate. A Phoenix lawn in July (ETo ≈ 2.5 in/week) needs about 2.1 inches/week gross from a sprinkler at 80% efficiency, while the same lawn in a Seattle winter may need no irrigation at all.

Irrigation Water Need Calculator — Quick & Free

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Walk-through

How to Use This Calculator

5 steps

1

Enter irrigated acreage

Enter the area in acres. This page has no square-foot or metric area toggle; convert your area before entering it.

2

Choose crop and growth stage

Crop type and Early/Mid-season/Late growth stage select the model’s crop coefficient Kc; they do not select soil or climate factors.

3

Enter weekly reference ET and rainfall

Reference ET and rainfall are inches per week. Crop ET is reference ET × Kc; rainfall is subtracted and net irrigation cannot fall below zero.

4

Select system and cost per acre-inch

Choose center pivot, drip, furrow or sprinkler. Gross depth divides net need by system efficiency. Water cost is dollars per acre-inch, not per 1,000 gallons.

5

Compare weekly estimates

Review gallons, acre-inches and weekly cost. The system comparison holds net need constant. The seasonal tab contains illustrative weekly snapshots, not a local weather forecast.

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Reference

Formula & Methodology

3 formulas

Crop and net irrigation depth

ETcrop = ETref × Kc; Net = max(0, ETcrop − Rain)

Reference ET and rainfall are inches per week. Crop ET is reference ET × Kc; rainfall is subtracted and net irrigation cannot fall below zero.

Gross depth and volume

Gross = Net / Efficiency; US gal = Acres × Gross × 27154

Efficiency is a fraction: center pivot 0.85, drip 0.95, furrow 0.65, sprinkler 0.80. The model uses 27,154 US gallons per acre-inch.

Weekly water cost

Weekly cost = Acres × Gross (in/week) × $/acre-inch

Weekly acre-inches equal area × gross depth. Multiply by the entered dollar cost per acre-inch; the rate changes cost without changing water volume.

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Glossary

Key Terms Explained

6 terms

Reference ET Evapotranspiration of a reference surface, entered as inches per week and multiplied by the selected crop coefficient.

Crop coefficient Kc A dimensionless crop/stage multiplier. Mid-season corn uses 1.20 in this model; coefficients are planning assumptions.

Net irrigation Crop ET minus the entered weekly rainfall, floored at zero; no soil storage or effective-rainfall loss model is applied.

Gross irrigation Net depth divided by system efficiency: the applied depth needed under the worksheet’s assumed losses.

Acre-inch One inch of water over one acre; this model uses a rounded conversion of 27,154 US gallons.

Weekly cost Weekly acre-inches equal area × gross depth. Multiply by the entered dollar cost per acre-inch; the rate changes cost without changing water volume.

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Scenarios

Real-World Examples

2 worked examples

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Mid-season corn

Result

83,059 US gal/week; $12.24/week

Crop type corn Irrigated area (acres) 2 acres Growth stage mid; Kc=1.20 Reference ET (in/week) 1.5 in/week Rainfall (in/week) 0.5 in/week Irrigation system center-pivot; efficiency=0.85 Water cost rate ($/acre-inch) $4 / acre-inch

Crop ET 1.80 in/week, net depth 1.30, gross 1.5294118; 83,059.29 US gal/week and $12.24/week. The rounded displayed depth should not be reused as a calculation input.

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Rain covers crop ET

Result

0 US gal/week; $0/week

Crop type corn Irrigated area (acres) 2 acres Growth stage mid; Kc=1.20 Reference ET (in/week) 1.5 in/week Rainfall (in/week) 2.0 in/week Irrigation system center-pivot; efficiency=0.85 Water cost rate ($/acre-inch) $4 / acre-inch

Crop ET is 1.80 in/week and rainfall is 2.00, so net and gross irrigation, gallons and weekly water cost are all zero. The model does not turn excess rain into a negative water credit.

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Reference

Cite This Calculator

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Deep Dive

Irrigation Water Need Calculator

3 chapters · 2 min read

Review gallons, acre-inches and weekly cost. The system comparison holds net need constant. The seasonal tab contains illustrative weekly snapshots, not a local weather forecast.

Choose crop and growth stage

Crop type and Early/Mid-season/Late growth stage select the model’s crop coefficient Kc; they do not select soil or climate factors. Reference ET and rainfall are inches per week. Crop ET is reference ET × Kc; rainfall is subtracted and net irrigation cannot fall below zero.

Select system and cost per acre-inch

Efficiency is a fraction: center pivot 0.85, drip 0.95, furrow 0.65, sprinkler 0.80. The model uses 27,154 US gallons per acre-inch. Weekly acre-inches equal area × gross depth. Multiply by the entered dollar cost per acre-inch; the rate changes cost without changing water volume.

Compare weekly estimates

This acreage/ET calculator does not size sprinkler heads, zones, emitter spacing or runtime. It has no soil or climate selector. Use measured system flow and local agronomic advice for scheduling. The seasonal tab scales both weekly ET and rainfall using a fixed illustrative monthly factor, then computes one weekly snapshot per month. Its aggregate sums those snapshots; it is not a measured annual usage total or a regional forecast.

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Questions

Frequently Asked Questions

6 questions

Which area unit should I enter?+

Enter the area in acres. This page has no square-foot or metric area toggle; convert your area before entering it.

How does rainfall affect irrigation?+

Reference ET and rainfall are inches per week. Crop ET is reference ET × Kc; rainfall is subtracted and net irrigation cannot fall below zero.

What changes when I select another system?+

Efficiency is a fraction: center pivot 0.85, drip 0.95, furrow 0.65, sprinkler 0.80. The model uses 27,154 US gallons per acre-inch.

Is the water rate per 1,000 gallons?+

Weekly acre-inches equal area × gross depth. Multiply by the entered dollar cost per acre-inch; the rate changes cost without changing water volume.

Does this page size heads, zones or runtime?+

This acreage/ET calculator does not size sprinkler heads, zones, emitter spacing or runtime. It has no soil or climate selector. Use measured system flow and local agronomic advice for scheduling.

Is the seasonal table a measured annual total?+

The seasonal tab scales both weekly ET and rainfall using a fixed illustrative monthly factor, then computes one weekly snapshot per month. Its aggregate sums those snapshots; it is not a measured annual usage total or a regional forecast.

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How to Use This Calculator

Enter irrigated acreage

Choose crop and growth stage

Enter weekly reference ET and rainfall

Select system and cost per acre-inch

Compare weekly estimates

Formula & Methodology

Key Terms Explained

Real-World Examples

Mid-season corn

Rain covers crop ET

Cite This Calculator

Irrigation Water Need Calculator

Choose crop and growth stage

Select system and cost per acre-inch

Compare weekly estimates

Frequently Asked Questions

Get a branded PDF of your results

Irrigation Water Need Calculator

Weekly ET vs Rainfall — 4-Week View

12-Month Irrigation Schedule

Irrigation System Comparison

Total Gallons per Week by System

How to Use This Calculator

Enter irrigated acreage

Choose crop and growth stage

Enter weekly reference ET and rainfall

Select system and cost per acre-inch

Compare weekly estimates

Formula & Methodology

Key Terms Explained

Real-World Examples

Cite This Calculator

Irrigation Water Need Calculator

Choose crop and growth stage

Select system and cost per acre-inch

Compare weekly estimates

Frequently Asked Questions

Get a branded PDF of your results

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